It is often preferable to enable a load user to choose and control the output (speed, intensity, etc.) of a load capable of a variable output, and at the same time limit the chosen output to a predetermined time interval. It is also often preferable that the time interval be fixed and out of the control of the load user. For example, the manufacturer of a vibrating bed may want to allow the customer to control the speed of the vibrator motor, but limit the desired output to a fixed time interval. Such a circuit essentially provides automatic timed turn off of the vibrator motor, and allows the customer to fall asleep without being concerned that the motor will run indefinitely. A hotel owner may also want to allow guests to control the intensity of a bathroom heater lamp, but limit operation of the lamp to a fixed, predetermined time period, thereby reducing energy expenses. Energy costs may also be reduced in public restrooms wherein the chosen light intensity is limited to a fixed, predetermined time interval. These energy benefits are achieved in the present invention by requiring that the load be reset at the termination of the fixed time interval in order to operate the load beyond the predetermined time limits. The invention circuit can also be utilized to control a timed hot plate or coffee maker and/or the water pressure solenoid valves in a car wash. Despite these beneficial applications, however, no previous known circuits have combined the simultaneous control of a variable output with fixed, predetermined timer limitations in a unitary circuit board.
The present invention provides a variable output control circuit wherein the chosen output is limited to a fixed, predetermined time interval and whereby the above described benefits can be achieved in a unitary circuit board. A timer chip having an RC network receives rectified current from an AC voltage source and, in response thereto, provides a high output for a fixed, predetermined time interval, thereby biasing an NPN transistor for the duration of the time interval. The biased transistor permits the flow of rectified current to a logic triac, thereby triggering the gate of the logic triac and permitting current flow from the AC voltage source to a variable resistor in series connection with a gating/AC timing capacitor, the controlled current thereby triggering a trigger diode or diac. The diac responds to the controlled current from the variable resistor by triggering a power control triac, the power control triac thereby permitting the desired flow of current between the AC voltage source and load through its terminals. At the termination of the fixed time interval, the timer chip output becomes low and ceases to bias the NPN transistor, thereby prohibiting the activation of the logic triac and the flow of current from the AC voltage source to the variable resistor and diac. The diac will likewise cease to trigger the power control triac, thereby prohibiting the flow of current between the AC voltage source and the AC load. As such, the desired flow of current to the load as determined by external input to the variable resistor and the corresponding output will only continue for the duration of the fixed, predetermined time interval.